This invention relates to a liquid crystal matrix device using a ferroelectric liquid crystal having a smectic phase, and more particularly to a liquid crystal display device suitable for large scale display.
Ferroelectric liquid crystal molecules assume a layered structure and a spiral structure such as shown in FIG. 2 of the accompanying drawings. In the drawings, reference numeral 1 represents liquid crystal molecules and 2 represents spontaneous polarization.
When an electric field E above a threshold voltage is applied vertically to a spiral axis, the molecules move inside the layer while keeping the layered structure and the spiral gets loosened so that a permanent dipole moment vertical to the long major axis of each molecule becomes parallel to the electric field. Accordingly, the molecules are oriented parallel to one another not only in the layers but also between the layers as shown in FIG. 2(a).
If the direction of the electric field is reversed, the liquid crystal molecules assume the state shown in FIG. 2(c). In other words, two states where the liquid crystal molecules are inclined by .+-..theta. can be established by selecting the direction of the electric field, and a display device or an optical shutter device can be produced by either utilizing birefringence or adding a dichroic pigment to the liquid crystal.
When the electric field is removed, the ferroelectric liquid crystal molecules generally return to the original spiral structure due to the orientation elastic righting moment as shown in FIG. 2(b), but it is known in the art that when the liquid crystal layer is as thin as about 1 .mu.m, for example, a bistable state where the spiral remains substantially loosened such as shown in FIGS. 2(a) and (c) can be established even when the field is zero.
One example of the conventional time-division driving methods of the ferroelectric liquid crystal exhibiting such a bistable state is shown in FIGS. 3 and 4.
FIG. 3 shows the outline of a liquid crystal device. A liquid crystal as a ferroelectric liquid crystal exhibiting a chiral smectic phase is sealed between X and Y electrodes 3 and 4.
FIG. 4 shows driving waveforms to be applied to the X and Y electrodes 3, 4 when a pixel A is turned ON while a pixel B is turned OFF.
A voltage having a voltage value of .+-.2 V is sequentially applied to the X electrode, while a voltage having a voltage value of .+-.V is applied to the Y electrode. As a result, the .+-.3 V voltage or .+-.V voltage is applied to the pixel A, which is turned ON, while the -3 V voltage or .+-.V voltage is applied to the pixel B, which is turned OFF.
In accordance with this driving method, the application time .DELTA.t of .+-.3 V voltage which determines the display state of the pixels is 1/4 of the selection time T.sub.s of one line. Therefore, the optical response time of the liquid crystal must be below 1/4 T.sub.s.
On the other hand, the optical response time of the smectic liquid crystals available at present is from about 0.5 to about 1 ms. Therefore, if the number of scanning lines is N=500, the re-write time of one picture surface is as long as about two seconds because the selection time T.sub.s of one line is T.sub.s =4 ms.
As the prior art references relating to the driving methods of the kind described above, mention can be made of Japanese Patent Laid-Open Nos. 123,825/1985 and 33535/1985.
Here, the driving method disclosed in Japanese Patent Laid-Open No. 123,825/1985 will be explained.
This driving method makes scanning twice, that is, ON scanning and OFF scanning, to re-write the display content of one picture surface. FIGS. 49(a) and 49(b) show the voltage waveforms to be applied to scanning electrode (common electrode) and to signal electrode (segment electrode) in ON and OFF scanning, respectively.
In the drawings, symbols .phi..sub.Yl, .phi..sub.Yl, .phi..sub.Yd and .phi..sub.Yd denote the scanning voltages to be applied to the scanning electrode while .phi..sub.Xl, .phi..sub.Xl, .phi..sub.Xd and .phi..sub.Xd represent the signal voltages to be applied to the signal electrode.
FIG. 50 shows the voltage which is determined from FIGS. 49(a) and (b) and applied to the liquid crystal. This voltage represents the waveform when the matrix liquid crystal consisting of the signal electrodes 301 and the scanning electrodes 302 shown in FIG. 51 is driven on the time division basis.
The voltage applied to a pixel 303a when setting the pixels 303a-303e to the display state shown in the drawing is V.sub.Yl -V.sub.Xl. Here, the display ON state is set when a negative voltage (-V.sub.ap) is applied to the liquid crystal.
As shown in the drawing, a .+-.1/3V.sub.ap bias voltage is applied during the non-selection period of the pixel 303a, but the application time of the same polarity is not constant but changes in two stages.
On the other hand, it is known that the optical threshold voltage of ferroelectric liquid crystals is not clear with respect to a d.c. voltage. Therefore, the liquid crystal responds to the bias voltage and the peak value of a transmission light quantity T becomes greater with a longer application time of the same polarity and becomes smaller with a shorter application time. As a result, during the re-write operation of information, variance occurs in the light transmission state for the reasons described above and the display quality deteriorates. In other words, flicker of the display occurs on a display and the display quality drops during the rewrite operation of the picture surface.
As described above, when applied to a large picture surface high precision liquid crystal panel having a large number of scanning lines, the conventional driving methods involve the practical problems that a long time is necessary for re-writing the entire picture surface and variance occurs in the light transmission state.